add -2 and -3 switch to run-tests.py

These switches prefix all subcommands with either "python2" or
"python3". This allows to run the tests on python2 and python3 if both
python versions are installed in one system. If those arguments are not
provided, the default python enviroment is used.

run-tests.py

@@ -50,22 +50,20 @@ def run_autopep8(args):
         print('$ ' + ' '.join(argv))
         autopep8.main(argv)
 
-def run_alltests():
+def run_alltests(python='python'):
     '''
     runs all tests on postpic. This function has to exit without error on every commit!
     '''
+    python += ' '
     # make sure .pyx sources are up to date and compiled
-    subprocess.call(os.path.join('.', 'setup.py develop --user'), shell=True)
-    # run nose tests
-    import nose
-    ex = nose.run()  # returns True on success
-    exitonfailure(not ex, cmd='nosetests')
+    subprocess.call(python + os.path.join('.', 'setup.py develop --user'), shell=True)
 
-    cmds = ['pep8 postpic --statistics --count --show-source '
+    cmds = [python + '-m nose',
+            python + '-m pep8 postpic --statistics --count --show-source '
             '--ignore=W391,E123,E226,E24 --max-line-length=99',
-            os.path.join('examples', 'simpleexample.py'),
-            os.path.join('examples', 'particleshapedemo.py'),
-            os.path.join('examples', 'time_cythonfunctions.py')]
+            python + os.path.join('examples', 'simpleexample.py'),
+            python + os.path.join('examples', 'particleshapedemo.py'),
+            python + os.path.join('examples', 'time_cythonfunctions.py')]
     for cmd in cmds:
         print('=====  running next command =====')
         print('$ ' + cmd)
@@ -84,13 +82,19 @@ def main():
         Use "--autopep8" to preview changes. To apply them, use
         "--autopep8 fix"
         ''')
+    pyversiongroup = parser.add_mutually_exclusive_group(required=False)
+    pyversiongroup.add_argument('-2', action='store_const', dest='pycmd', const='python2',
+                                help='Prefix all subcommands with "python2"',
+                                default='python')
+    pyversiongroup.add_argument('-3', action='store_const', dest='pycmd', const='python3',
+                                help='Prefix all subcommands with "python3"')
     args = parser.parse_args()
 
     if args.autopep8 != '':
         run_autopep8(args)
         exit()
 
-    run_alltests()
+    run_alltests(args.pycmd)
 
 if __name__ == '__main__':
     main()